A model system that consists of a muscle utilized in biting, the accessory radula closer (
ARC), and the two
cholinergic motor neurons innervating this muscle, neurons B15 and B16, has been used to study the expression of food-induced arousal in the marine mollusk Aplysia. The
ARC muscle receives modulatory input from an extrinsic source, the serotonergic metacerebral cells, which partially accounts for the progressive increase in the strength of biting seen in aroused animals. Another source of modulation may arise from the
ARC motor neurons themselves, which synthesize
neuropeptides that can potentiate
ARC contractions. Neuron B15 synthesizes the two homologous
peptides, small cardioactive
peptides A and B, whereas neuron B16 synthesizes the structurally unrelated
peptide myomodulin. Here we report the purification and sequencing of a
neuropeptide termed
buccalin and show that it is colocalized with the small cardioactive
peptides to neuron B15.
Buccalin is also bioactive at the
ARC neuromuscular junction but, in contrast to the small cardioactive
peptides, when exogenously applied, it decreases rather than increases the size of muscle contractions elicited by firing of the motor neurons. Also unlike the small cardioactive
peptides, which exert postsynaptic actions,
buccalin seems to act only presynaptically. It has no effect on muscle relaxation rate and decreases motor neuron-elicited excitatory junction potentials in the
ARC without affecting contractions produced by direct application of
acetylcholine to the muscle. Neuron B15, therefore, appears to contain three modulatory
neurotransmitters, two of which may act postsynaptically on the muscle to potentiate the action of the primary
neurotransmitter acetylcholine and one of which may act presynaptically on nerve terminals to inhibit
acetylcholine release.